Visiting Norway to catch the aurora borealis has recently grown in popularity, and is making its way into the bucket list of many. From the encounters of those who have seen it, it is truly a magical experience! To these lucky few, no videos or pictures could do justice to the actual experience of being under the vast open night skies, lit ablaze by the tapestry of colours.

h2 physics tutor - aurora display

However, one might wonder, how did the aurora come about? As it turns out, the beautiful display of the aurora borealis is actually formed thanks to the Sun! Interestingly, despite the Sun not being present in the night sky, the effects of its invisible plasma still manifest in the aurora.

To understand how the aurora works, we must first start with the solar wind. This is a cloud of plasma, comprising of mainly electrons, which is being spewed out from the Sun constantly. Eventually, some of this solar wind finds itself in the Earth’s magnetic field, also called the magnetosphere. As the two interact, the magnetosphere is compressed on the day side, and is stretched into a long magnetotail much like the wake of a speedboat on the night side.

h2 physics tutor - magnetosphere

Artist rendition of magnetosphere – compressed on the day side and stretched on the night side

This compressing and stretching process stores electrons in the magnetotail, thus building up a potential difference between the tail and the poles. This is akin to the potential difference stored between a battery. Recalling chapter 12 on Electric Field, this will then cause the electrons to accelerate towards the Earth’s North and South poles as electric potential energy is converted to kinetic energy, much like how the potential difference in a battery causes the electrons to flow in the circuit, firing up electrical components along the way.

As we have learnt in Electromagnetism, the negatively-charged electrons will be deflected by the Earth’s magnetic field into a circular path. This combined acceleration towards the poles and circular path thus result in the electrons moving in a helical path (although it is not a perfect circle due to winds in the atmosphere). Finally, as the fast-moving electrons collide violently with the air particles, the kinetic energy of the electrons is lost and emitted as photons, or light energy*. As such, a helical ring of light is formed up in the sky. From the ground, what we see as resembling curtains of light rising up in the sky is actually part of a helix cascading towards the Earth. From an extraterrestrial vantage point, we can see this circular path clearly.

h2 physics tutor - aurora helix

So, in future if you do get to see the aurora, remember that you are not alone. In fact, everyone who is under that ring of light can see the same magical display as well! đŸ™‚



* Finally, we will learn how electrons colliding with gas atoms can convert kinetic energy to photons in a later chapter on Quantum Physics!